Image capturing apparatus and control method for the same

ABSTRACT

An image capturing apparatus includes an image sensor having an effective pixel area composed of effective pixels that photoelectrically convert an object image and a reference pixel area composed of reference pixels that output pixel signals to be a reference; a first calculation unit that calculates a standard deviation of the pixel signals output from the reference pixel area; a second calculation unit that calculates an integrated value of the pixel signals output from the reference pixel area; a third calculation unit that calculates a correction value for correcting pixel signals output from the effective pixel area, with use of the integrated value; a correction unit that corrects the pixel signals output from the effective pixel area, based on the correction value; and a determination unit that determines whether correction is to be performed by the correction unit, according to the standard deviation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage application of InternationalApplication No. PCT/JP2010/056275, filed Mar. 31, 2011, whose benefit isclaimed and which claims the benefit of Japanese Patent Application No.2009-114979, filed May 11, 2009, the entire disclosures of which areincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to noise correction in an image capturingapparatus, and in particular relates to the correction of stripe noise.

BACKGROUND ART

In recent years, CMOS image sensors have often been used in digitalsingle-lens reflex cameras and video cameras. An increase in the numberof pixels, an increase in image capturing speed, and an increase in ISOspeed (an improvement of sensitivity) have been required for such CMOSimage sensors.

Pixel size tends to become smaller due to an increase in the number ofpixels, and this means that less electric charge can be accumulated ineach pixel. Meanwhile, in order to accommodate an increase in ISO speed,a larger gain needs to be applied to the obtained electric charge.Although the original optical signal component is amplified when gain isapplied, noise generated by circuits and the like is also amplified, andtherefore high ISO speed images have more random noise than low ISOspeed images.

Also, one method of realizing high-speed image capturing ismultichannelization in which the image sensor is provided with aplurality of output paths, and readout is performed simultaneously for aplurality of pixels. However, since the amount of noise varies dependingon the output path, there is the problem that the amount of noisediffers for each CH (for each channel).

Below is a description of the configuration of a CMOS image sensor andthe cause of noise generation. FIG. 9 shows an overall layout of theCMOS image sensor. As shown in FIG. 9, the CMOS image sensor includes anaperture pixel area (effective pixel area) 903 having aperture pixels(effective pixels), and a vertical optical black area (VOB, firstreference pixel area) 902 and a horizontal optical black area (HOB,second reference pixel area) 901 that have shielded pixels (referencepixels). The HOB 901 is provided adjacent to the head (on the left side)of the aperture pixel area 903 in the horizontal direction, and is anarea shielded so that light does not enter. Also, the VOB 902 isprovided adjacent to the head (on the top side) of the aperture pixelarea 903 in the vertical direction, and is an area shielded so thatlight does not enter. The aperture pixel area 903 and the optical blackareas 901 and 902 have the same structure, and the aperture pixel area903 is not shielded, whereas the optical black areas 901 and 902 areshielded. Hereinafter, the pixels in the optical black areas are calledOB pixels. Normally, OB pixels are used to obtain a reference signalwhose signal level is a reference, that is to say a black referencesignal. The aperture pixels of the aperture pixel area 903 eachaccumulate an electric charge generated according to incident light, andoutput the electric charge.

FIG. 10 shows an example of a circuit of a unit pixel (corresponding toone pixel) in the CMOS image sensor. A photodiode (hereinafter, called aPD) 1001 receives an optical image formed by an imaging lens, generatesan electric charge, and accumulates the electric charge. Referencenumeral 1002 indicates a transfer switch that is configured by a MOStransistor. Reference numeral 1004 indicates a floating diffusion(hereinafter, called an FD). The electric charge accumulated by the PD1001 is transferred to the FD 1004 via the transfer MOS transistor 1002,and then converted to a voltage and output from a source followeramplifier 1005. Reference numeral 1006 indicates a selection switch thatcollectively outputs one row-worth of pixel signals to a vertical outputline 1007. Reference numeral 1003 indicates a reset switch that, withuse of a power source VDD, resets the potential of the FD 1004, andresets the potential of the PD 1001 via the transfer MOS transistor1002.

FIG. 11 is a block diagram showing an exemplary configuration of a CMOSimage sensor. Note that although FIG. 11 shows a 3×3 pixelconfiguration, normally the number of pixels is high, such as severalmillions or several tens of millions. A vertical shift register 1101outputs signals from row select lines Pres1, Ptx1, Psel1, and the liketo a pixel area 1108. The pixel area 1108 has the configuration shown inFIG. 9, and has a plurality of pixel cells Pixel. Even-numbered columnsand odd-numbered columns of the pixel cells Pixel output pixel signalsto vertical signal lines of a CH1 and a CH2 respectively. A constantcurrent source 1107 is connected as a load to the vertical signalslines. A readout circuit 1102 receives an input of a pixel signal from avertical signal line, outputs the pixel signal to a differentialamplifier 1105 via an n-channel MOS transistor 1103, and outputs a noisesignal to the differential amplifier 1105 via an n-channel MOStransistor 1104. A horizontal shift register 1106 controls the switchingon/off of the transistors 1103 and 1104, and the differential amplifier1105 outputs a difference between the pixel signal and the noise signal.Note that although the output path configuration in FIG. 11 is atwo-channel configuration including CH1 and CH2, high-speed processingis made possible by increasing the number of output paths. For example,if a total of eight output paths (in other words, four output paths bothabove and below in the image sensor configuration) are provided, eightpixels can be processed at the same time.

Using the differential amplifier described above enables obtaining anoutput signal from which noise unique to the CMOS image sensor has beenremoved. However, if there is variation between the characteristics ofthe output amplifiers of CH1 and CH2, a substantially uniform leveldifference occurs in each column. This is called vertical pattern noise.

Meanwhile, the pixels have a common power source and GND. If the powersource and GND fluctuate during a readout operation, the pixels read outat that time have a substantially uniform level difference. Normally,readout is performed in an image sensor row-by-row, from left to right,beginning at the top left of the screen. The level difference occurringdue to fluctuation of the power source and the GND appears as adifferent level difference for substantially each row. This is calledhorizontal pattern noise.

As described above, there is the problem that stripe noise occurs due tothe structure of the CMOS image sensor, and this stripe noise tends tobe more prominent as the specifications are improved. Since the verticalpattern noise is unique pattern noise determined by the characteristicsof the output amplifiers, correction can be performed by correctingvariations in each output amplifier. On the other hand, if thefluctuation of the power source and the GND is random, the horizontalpattern noise also becomes random.

As a technique for correcting such random pattern noise, Japanese PatentLaid-Open No. 7-67038 discloses a method of calculating a line averagevalue for pixel signals of OB pixels, and subtracting the line averagevalue from the pixel signals of aperture pixels in that row.

However, in an image that has a large amount of random noise,calculating a correction value for correcting stripe noise is difficult.This is pointed out in Japanese Patent Laid-Open No. 7-67038, JapanesePatent Laid-Open No. 2005-167918, and the like as well. According toJapanese Patent Laid-Open No. 2005-167918, if the stripe noise isreduced to from ⅛ to 1/10 of the random noise, the stripe noise becomesburied in the random noise, and thus becomes difficult to see. In viewof this, Japanese Patent Laid-Open No. 2005-167918 discloses a method inwhich noise is mitigated by adding random noise.

However, with the correction method of subtracting the line averagevalue of pixel signals of OB pixels, there are often cases in which thecorrection is insufficient, such as the case in which stripe noiseoccurs due to under-correction and over-correction. This phenomenon canoften be seen in images containing a large amount of random noise, suchas images captured at a high ISO speed. As the amount of random noise inan image rises, more OB pixels are necessary to obtain a propercorrection value. Also, even given that the stripe noise is difficult tosee if it is ⅛ to 1/10 or less of the random noise value as disclosed inthe above-mentioned Japanese Patent Laid-Open No. 2005-167918,calculating a correction value such that the stripe noise becomesdifficult to see necessitates approximately 400 or more OB pixels perrow. However, allocating 400 columns or more to OB pixels in the layoutof the CMOS image sensor cannot be said to be practical in view of therequirement for an increase in the number of pixels and high-speedimaging.

SUMMARY OF INVENTION

The present invention has been achieved in light of the issues describedabove, and enables effectively correcting horizontal stripe noise evenin the case in which there are few reference pixels.

According to a first aspect of the present invention, an image capturingapparatus includes: an image sensor having an effective pixel areacomposed of effective pixels that photoelectrically convert an objectimage, and a reference pixel area composed of reference pixels thatoutput pixel signals to be a reference; a correction means forcorrecting pixel signals output from the effective pixel area with useof a correction value calculated based on the pixel signals output fromthe reference pixel area; and a determination means for determiningwhether correction is to be performed by the correction means, inaccordance with values of a statistical measure of the pixel signalsoutput from the reference pixel area.

Also, according to a second aspect of the present invention, a controlmethod for an image capturing apparatus is a control method for an imagecapturing apparatus provided with an image sensor having an effectivepixel area composed of effective pixels that photoelectrically convertan object image, and a reference pixel area composed of reference pixelsthat output pixel signals to be a reference, the control methodincluding the steps of: calculating values of a statistical measure ofthe pixel signals output from the reference pixel area; calculating acorrection value for correcting pixel signals output from the effectivepixel area, based on the pixel signals output from the reference pixelarea; correcting the pixel signals output from the effective pixel areawith use of the correction value; and determining whether correction isto be performed in the correction step, according to the values of astatistical measure.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an overall block diagram showing a configuration of an imagecapturing apparatus according to Embodiment 1 of the present invention.

FIG. 2 is a cross-sectional view of a CMOS image sensor.

FIG. 3 is a diagram showing an example of a circuit corresponding to onecolumn in a readout circuit block shown in FIG. 5.

FIG. 4 is a timing chart showing an example of operations performed bythe CMOS image sensor.

FIG. 5 is a diagram showing an example of an image obtained by the imagecapturing apparatus.

FIG. 6 is flowchart of horizontal stripe noise correction processingaccording to Embodiment 1 of the present invention.

FIG. 7 is flowchart of horizontal stripe noise correction processingaccording to Embodiment 2 of the present invention.

FIG. 8 is flowchart of horizontal stripe noise correction processingaccording to Embodiment 3 of the present invention.

FIG. 9 is a diagram showing an overall layout of the CMOS image sensor.

FIG. 10 is a diagram showing an example of a circuit of a unit pixel(corresponding to one pixel) in the CMOS image sensor.

FIG. 11 is a block diagram showing an exemplary configuration of a CMOSimage sensor.

DESCRIPTION OF EMBODIMENTS

Below is a detailed description of embodiments of the present inventionwith reference to the drawings.

Embodiment 1

FIG. 1 is an overall block diagram showing a configuration of an imagecapturing apparatus according to Embodiment 1 of the present invention.In FIG. 1, an image sensor 101 is a CMOS image sensor thatphotoelectrically converts an object image formed by an imaging lens(not shown). An AFE 102 is an analog front end, which is a signalprocessing circuit that performs amplification, black level adjustment(OB clamp), and the like on signals from the image sensor 101. The AFE102 receives an OB clamp timing, an OB clamp target level, and the likefrom a timing generation circuit 110, and performs processing inaccordance with these. The AFE 102 also converts processed analogsignals into digital signals. A DFE 103 is a digital front end thatreceives digital signals of pixels obtained by the conversion performedby the AFE 102, and performs digital processing such as image signalcorrection and pixel rearrangement. Reference numeral 105 indicates animage processing apparatus that performs developing processing, and alsoprocessing such as displaying an image on a display circuit 108 andrecording an image to a recording medium 109 via a control circuit 106.Note that the control circuit 106 also receives instructions from acontrol unit 107 and performs control such as sending instructions tothe timing generation circuit 110. Also, a CompactFlash (registeredtrademark) memory or the like is used as the recording medium 109. Amemory circuit 104 is used as a work memory in the developing stage inthe image processing apparatus 105. The memory circuit 104 is also usedas a buffer memory for when image capturing is performed in successionand developing processing is not completed on time. The control unit 107includes, for example, a power source switch for starting a digitalcamera, and a shutter switch that instructs the start of imagingpreparation operations such as photometric processing and rangingprocessing, and the start of a series of image capturing operations fordriving a mirror and a shutter, processing signals read out from theimage sensor 101, and writing the resulting signals to the recordingmedium 109.

The configurations of pixel areas of the image sensor 101 are similar tothe configurations in FIG. 9, and specifically the image sensor 101includes an aperture pixel area (effective pixel area) 903 havingaperture pixels (effective pixels), and a vertical optical black area(VOB, first reference pixel area) 902 and a horizontal optical blackarea (HOB, second reference pixel area) 901 that have shielded pixels(reference pixels) that are shielded such that light does not enter.

FIG. 2 is a cross-sectional view of the CMOS image sensor. An AL1, anAL2, and an AL3 (205, 204, and 203 in FIG. 2) are wiring layers, and areconfigured by aluminum or the like. The AL3 (203) is also used for lightshielding, and a pixel 1 and a pixel 2, which are OB pixels, areshielded by the AL3. A pixel 3 and a pixel 4, however, are not shieldedby the AL3, and are aperture pixels. MLs (201) are microlenses thatconverge light onto photodiodes PD (207). CFs (202) are color filters.PTXs (206) are transfer switches that transfer electric chargeaccumulated in the PDs (207) to FDs (208).

The circuit configuration of a unit pixel (corresponding to one pixel)of the CMOS image sensor according to the present embodiment is similarto the configuration in FIG. 10, and therefore a detailed descriptionthereof has been omitted. The overall configuration of the CMOS imagesensor according to the present embodiment is similar to theconfiguration in FIG. 11.

The gate of a transfer MOS transistor 1002 in FIG. 10 is connected to afirst row select line Ptx1 (FIG. 11) disposed extending in thehorizontal direction. The gates of similar transfer MOS transistors 1002of other pixel cells Pixel disposed in the same row are also connectedto the first row select line Ptx1 in common. The gate of a reset MOStransistor 1003 in FIG. 10 is connected to a second row select linePres1 (FIG. 11) disposed extending in the horizontal direction. Thegates of similar reset MOS transistors 1003 of other pixel cells Pixeldisposed in the same row are also connected to the second row selectline Pres1 in common. The gate of a select MOS transistor 1006 in FIG.10 is connected to a third row select line Psel1 disposed extending inthe horizontal direction. The gates of similar select MOS transistors1006 of other pixel cells Pixel disposed in the same row are alsoconnected to the third row select line Psel1 in common, and the first tothird row select lines Ptx1, Pres1, and Psel1 are connected to avertical shift register 1101, and are thus driven.

Pixel cells Pixel and row select lines having a similar configurationare provided in the remaining rows shown in FIG. 11 as well. These rowselect lines include row select lines Ptx2 and Ptx3, Pres2 and Pres3,and Psel2 and Psel3, which are formed by the vertical shift register1101.

The source of the select MOS transistor 1006 is connected to a terminalVout of a vertical signal line disposed extending in the verticaldirection. The source of similar select MOS transistors 1006 of pixelcells Pixel disposed in the same column is also connected to theterminal Vout of the vertical signal line. In FIG. 11, the terminal Voutof the vertical signal line is connected to a constant current source1107, which is a load.

FIG. 3 is a diagram showing an example of a circuit corresponding to onecolumn in the readout circuit 1102 block shown in FIG. 11. The portionenclosed in dashed lines is the portion corresponding to the column, andthe terminal Vout is connected to each vertical signal line.

FIG. 4 is a timing chart showing an example of operations performed bythe CMOS image sensor. The gate line Pres1 of the reset MOS transistor1003 changes to the high level prior to the readout of the signalelectric charge from the photodiode 1001. Accordingly, the gate of theamplification MOS transistor is reset to a reset power source voltage.At the same time as the gate line Pres1 of the reset MOS transistor 1003returns to the low level, a gate line Pc0r (FIG. 3) of a clamp switchchanges to the high level, and thereafter the gate line Psel1 of theselect MOS transistor 1006 changes to the high level. Accordingly, resetsignals (noise signals) having reset noise superimposed thereon are readout to the vertical signal line Vout, and clamped by clamp capacitors COin the columns. Next, the gate line Pc0r of the clamp switch returns tothe low level, and thereafter a gate line Pctn of a transfer switch onthe noise signal side changes to the high level, and the reset signalsare held in noise holding capacitors Ctn provided in the columns. Next,a gate line Pcts of a transfer switch on the pixel signal side ischanged to the high level, and thereafter the gate line Ptx1 of thetransfer MOS transistor 1002 changes to the high level, and the signalelectric charge of the photodiode 1001 is transferred to the gate of asource follower amplifier 1005 and also read out to the vertical signalline Vout at the same time. Next, the gate line Ptx1 of the transfer MOStransistor 1002 returns to the low level, and thereafter the gate linePcts of the transfer switch on the pixel signal side changes to the lowlevel. Accordingly, changed portions (optical signal components) fromthe reset signals are read out to signal holding capacitors Cts providedin the columns. As a result of the operations up to this point, thesignal electric charges of the pixels Pixel connected in the first roware held in the signal holding capacitors Ctn and Cts connected in therespective columns.

Next, the gates of horizontal transfer switches in the columnssequentially change to the high level in accordance with signals Phsupplied from a horizontal shift register 1106. The voltages held in thesignal holding capacitors Ctn and Cts are sequentially read out byhorizontal output lines Chn and Chs, difference processing is performedthereon by an output amplifier, and the resulting signals aresequentially output to an output terminal OUT. During the signal readoutperformed in each column, the horizontal output lines Chn and Chs arereset to reset voltages VCHRN and VCHRS by a reset switch. Thiscompletes the readout of the pixel cells Pixel connected in the firstrow. Subsequently, in a similar manner, the signals of the pixel cellsPixel connected in the second row and rows thereafter are sequentiallyread out in accordance with signals from the vertical shift register1101, and thus the readout of all the pixel cells Pixel is completed.

FIG. 5 shows an example of images obtained by the processing describedabove. There is a time difference between Pctn and Pcts, and if thepower source and GND fluctuate during such time, the signal level of theentire row uniformly changes. Horizontal stripe noise appears since suchfluctuation is different for each row. Since more gain is supplied whenhigh ISO speed imaging is performed (when high sensitivity imaging isperformed), the noise is also amplified, and therefore the horizontalstripe noise becomes prominent.

FIG. 6 is flowchart of horizontal stripe noise correction processingaccording to Embodiment 1 of the present invention. The following is adescription of the stripe noise correction method according to thepresent embodiment with reference to this flowchart. Note that thedescription in the present embodiment is based on the assumption thatcorrection is performed after acquiring an image that has not beendeveloped yet. The correction value and correction coefficient thatappear in the following description are defined as follows. Thecorrection value is a value obtained from HOB signals for each row, andthe pixels in each row are corrected in accordance with expressions thatare described later. Here, the correction coefficient is defined as acoefficient by which a shift amount is multiplied, where the shiftamount is an amount of shift from a black reference value calculatedfrom the HOB signals.

First, readout is started in step S601. The readout is performed fromleft to right row-by-row, starting at the upper left of the pixelconfiguration layouts shown in FIGS. 5 and 9. The VOB area is providedat the top of the screen in the pixel configurations in FIGS. 5 and 9,and first a standard deviation σ_(VOB) of pixel signals output from theVOB area is calculated (step S602) (first calculation step). Althoughthe pixel area targeted for calculation may be any area as long as OBpixels are included, it is better for the calculation to be performedusing pixel signals from as many pixels as possible (first predeterminedarea) in order to properly determine the state of the image. As a resultof selecting many pixels as the area, σ_(VOB) is substantially equal tothe standard deviation σ of the overall image (The same applies toσ_(HOB) as well. In other words, σ_(OB)≈σ_(HOB)σ of overall image.).

Next, in step S603 a determination is made as to whether correction isto be performed, based on the calculated value of σ_(VOB). If σ_(VOB) isless than or equal to σ_(th) _(—) _(VOB), which is a threshold value setin advance, processing proceeds to step S604, and correction isperformed. If σ_(VOB) is greater than σ_(th) _(—) _(VOB), processingproceeds to step S609, and correction is not executed. The reason forthis is that if σ of the image (here, σ_(VOB)) is high, properlyobtaining the correction value is difficult, and there is the risk ofperforming erroneous correction, that is to say, increasing the amountof noise.

If σ_(VOB) is less than or equal to the threshold value, processingproceeds to step S604. A correction coefficient α is determinedaccording to the value of σ_(VOB). Normally, since erroneous correctiontends to be performed when the amount of shift from the black referencevalue calculated from the HOB signals is set as the correction value, afavorable correction result is obtained by setting the correctioncoefficient to a value of 1 or less, determining the correction value,and then executing correction. In particular, there is a strongertendency for over-correction to be performed as the number of columns inthe HOB decreases, or as the amount of random noise in the imageincreases. In other words, it is desirable for α to be lower as σ_(VOB)is higher (A). For example, if σ_(VOB) is 40, then α is set to 0.5, andif σ_(VOB) is 20, α is set to 0.7. Furthermore, the correctioncoefficient α may be caused to reflect the width of the HOB as well (B).For example, in the case in which σ_(VOB) is 40, α can be set to 0.5 ifthe width of the HOB is 100, and to 1.0 if the width of the HOB is 400.The correction coefficient α may be a table or function in the case of(A) and (B).

In step S605, in order to determine the correction value for an i-throw, an integrated value S_(i) of pixel signals output from the HOB(second predetermined area) is calculated (i being a verticalcoordinate) (second calculation step). Since correction only needs to beperformed for effective pixels, step S605 may begin to be executed whenthe readout row reaches the effective pixel area. Also, in the case inwhich there are multiple channels as the output paths as in FIG. 11, anintegrated value of pixel signals output from the HOB may be calculatedfor each output path, or since horizontal stripes are constant for eachrow regardless of the CH (channel) and color, an integrated value of allpixel signals output from HOB pixels in a row, regardless of the outputpath, may be calculated. Alternatively, in consideration of simplifyingthe calculation and the like, an integrated value may be calculated foreach of the colors R, G, and B.

Next, in step S606 a correction value V_(i) for the i-th row isdetermined (i being a vertical coordinate). The correction value iscalculated according to expression (1) (third calculation step).Specifically, an average value is calculated by dividing the integratedvalue calculated in step S605 by the number of data pieces used in thecalculation of the integrated value, and then a black reference levelset in advance is subtracted from the average value. The result is thenmultiplied by the correction coefficient α determined in step S604, thusobtaining the correction value for that row.correction value V _(i)=α×(S _(i)/number of data pieces−black referencevalue)  (1)

Then, in step S607, correction is performed on an effective pixel unitin the i-th row in accordance with Expression (2), with use of thecorrection value V_(i).corrected pixel signal x′(j,i)=pixel signal (j,i)−correction valueV_(i)(j being a horizontal coordinate)  (2)

The processing for the row ends when the effective pixel signalcorrection calculation has been performed through to the end of the row.Processing then returns to step S605, and this processing is repeatedthrough to the last row of the image (step S608).

Performing the processing described above, a correction value thatreflects the state of random noise in an image is determined, whichenables the execution of stripe noise correction without newlyincreasing the amount of noise. In other words, when there is a largeamount of random noise in an image, correction is not performed, orprocessing in which the correction amount is reduced by reducing thecorrection coefficient is executed, thus enabling the execution ofstripe noise correction without newly increasing the amount of noise.The reason for this is that a situation in which the amount of noise isnewly increased often occurs in the case in which the correction valueis larger than the proper correction value, and due to a large amount ofrandom noise, and the present embodiment solves this issue.

Note that although an integrated value of the HOB is calculated usingonly the signals of pixels in rows on which correction is to beperformed in step S605 in the present embodiment, the integrated valuemay be calculated using several rows of HOB pixels in higher/lower rows.Also, occasionally there are cases in which a pixel with an abnormallyhigh or low signal exists. A more proper correction value can becalculated by adding processing such as clipping such a pixel to acertain level before calculating the integrated value, or not using(skipping) such a pixel in the calculation of the integrated value.

Also, although a description is given in the present embodiment in whichcorrection processing is performed after acquiring an image, suchprocessing may be performed in the AFE 102 at the same time as readout.

Embodiment 2

The following describes Embodiment 2 of the present invention withreference to the flowchart shown in FIG. 7. Note that the processing upto and including the acquisition of an image that has not been developedyet is similar to that in Embodiment 1, and therefore a descriptionthereof has been omitted.

First, readout is started in step S701. Similarly to Embodiment 1, thereadout is performed from left to right row-by-row, starting at theupper left of the pixel configuration layouts shown in FIGS. 5 and 9.Next, the standard deviation σ_(VOB) of pixel signals output from theVOB area is calculated (step S702). Similarly to Embodiment 1, althoughthe pixel area targeted for calculation may be any area as long as OBpixels are included, it is better for the calculation to be performedusing as many pixels as possible in order to properly determine thestate of the image.

Next, in step S703 the integrated value S_(i) of pixel signals outputfrom the HOB pixels in the i-th row is calculated (i being a verticalcoordinate, for example, i=0,1,2, . . . ,n). Note that in order toprevent abnormal data from being used in the integrated valuecalculation, for example, if a pixel signal is higher than the blackreference level ±256, the pixel signal is clipped to the black referencelevel +256, and if lower than the black reference level ±256, the pixelsignal is clipped to the black reference level −256. The integratedvalue S_(i) is held in a memory, and integrated value calculation isexecuted through to the last row of the image. Also, although anintegrated value is calculated for a row of HOB pixels regardless of theoutput path or color in order to simplify the description here, thepixel signals may be demultiplexed into channels before executing theintegrated value calculation. Also, in the calculation of the integratedvalue S_(i) of the pixel signals output from the HOB pixels in the i-throw, not only the pixel signals output from the HOB pixels in the i-throw, but also signals output from HOB pixels in several higher/lowerrows may be used.

In step S704, a standard deviation σ_(Vline) of integrated values fromS0 to Sn of the pixel signals output from the HOB that were calculatedin step S703 is obtained (fourth calculation step). Next, adetermination is made as to whether correction is to be performed (stepS705), based on the value of σ_(VOB) obtained in step S702 and the valueof σ_(Vline) obtained in step S704. The calculation σ_(Vline/σ) _(VOB)is performed, and if the result is greater than or equal to adetermination value K, processing proceeds to step S706, and correctionis executed. If the result is less than the determination value K,processing proceeds to step S710, and correction is not executed. Here,σ_(Vline) reflects the magnitude and amount of stripe noise in theimage, and if σ_(Vline) is approximately greater than or equal to 0.1times σ_(VOB), which reflects the random noise component of the image,stripe noise can be confirmed visually as well, and therefore correctionis executed. On the other hand, if σ_(Vline) is less than 0.1 timesσ_(VOB), the stripe noise is not prominent. In this case, correction isnot executed since there is the risk of undesirably creating stripenoise if correction is executed.

In the case of executing correction, first the correction coefficient αis determined according to the value of σ_(VOB) (step S706). Thecorrection coefficient is calculated similarly to as in Embodiment 1.Next, in step S707 the correction value V_(i) for the i-th row isdetermined (i being a vertical coordinate). The correction value iscalculated in accordance with Expression (1) shown in Embodiment 1.Specifically, an average value is calculated by dividing the integratedvalue calculated in step S704 by the number of data pieces used in thecalculation of the integrated value, and then a black reference levelset in advance is subtracted from the average value. The result is thenmultiplied by the correction coefficient α determined in step S706, thusobtaining the correction value for that row.

Then, in step S708, correction is performed on an effective pixel unitin the i-th row in accordance with Expression (2), with use of thecorrection value V_(i).

The processing for the row ends when the effective pixel signalcorrection calculation has been performed through to the end of the row.Processing then returns to step S707, and this processing is repeatedthrough to the last row (step S709).

By performing the processing described above, the correction amount isadjusted in consideration of the magnitude of random noise similarly toEmbodiment 1, thus enabling performing horizontal stripe noisecorrection without newly increasing the amount of noise. Furthermore,since the state of horizontal stripe noise in an image is determinedbefore correction is performed, unnecessary processing is prevented frombeing performed when the amount of stripe noise is small with respect tothe image (in other words, is not prominent).

Embodiment 3

The following describes Embodiment 3 of the present invention withreference to the flowchart shown in FIG. 8. Note that the processing upto and including readout for all pixels is similar to that inEmbodiment 1. Also, the processing up to and including the determinationof whether to execute correction is substantially the same as inEmbodiment 2.

First, readout is started in step S801. Similarly to Embodiment 1, thereadout is performed from left to right row-by-row, starting at theupper left of the pixel configuration layouts shown in FIGS. 5 and 9.Next, the standard deviation σ_(VOB) of pixel signals output from theVOB area is calculated (step S802). Similarly to Embodiment 1, althoughthe pixel area targeted for calculation may be any area as long as OBpixels are included, it is better for the calculation to be performedusing as many pixels as possible in order to properly determine thestate of the image.

Next, in step S803 the integrated value S_(i) and standard deviationσ_(i) of pixel signals output from HOB pixels in the i-th row arecalculated (i being a vertical coordinate, for example, i=0,1,2, . . .,n). Note that in order to prevent abnormal data from being used in thecalculation of the integrated value and the standard deviation, forexample, if a pixel signal is higher than the black reference level±256, the pixel signal is clipped to the black reference level +256, andif lower than the black reference level ±256, the pixel signal isclipped to the black reference level −256. The integrated value S_(i)and the standard deviation σ_(i) are held in a memory, and theintegrated value calculation is executed through to the last row of theimage. Also, although an integrated value is calculated for a row of HOBpixels regardless of the output path or color in order to simplify thedescription here, the pixel signals may be demultiplexed into channelsbefore executing the integrated value calculation. Also, in thecalculation of the integrated value S_(i) and the standard deviationσ_(i) of signals of the i-th row, not only the pixel signals output fromthe HOB pixels in the i-th row, but also signals output from HOB pixelsin several higher/lower rows may be used.

In step S804, the Sn standard deviation σ_(Vline) is obtained from theintegrated values S0 of the pixel signals output from the HOB that werecalculated in step S803. Next, a determination is made as to whethercorrection is to be performed (step S805), based on the value of σ_(VOB)obtained in step S802 and the value of σ_(Vline) obtained in step S804.The calculation σ_(Vline)/σ_(VOB) is performed, and if the result isgreater than or equal to the determination value K, processing proceedsto step S806, and correction is executed. If the result is less than thedetermination value K, processing proceeds to step S810, and correctionis not executed.

In the case in which correction is to be executed, processing proceedsto step S806. A correction coefficient α_(i) is calculated for each rowto be corrected (i being a vertical coordinate, for example, i=0,1,2, .. . ,n). The correction coefficient α_(i) for the i-th row is calculatedusing σ_(i) that was calculated in step S803. If σ_(i) is high, thedegree of reliability is low since the integrated value S_(i) of HOBpixels in the i-th row has been calculated using pixel data thatincludes a large amount of variation. For this reason, the correctioncoefficient α_(i) is set to a low value. On the other hand, if σ_(i) islow, the degree of reliability is high since the integrated value S_(i)of HOB pixels in the i-th row has been calculated using pixel data thatincludes little variation. For this reason, the correction coefficientα_(i) is set to 1, or a number less than 1, but close to 1. Thecorrection coefficient α_(i) for the i-th row may be determined using atable with respect to σ_(i), and furthermore a configuration is possiblein which σ_(VOB) is added as a parameter, and the coefficient α_(i) forthe i-th row is determined using a function of σ_(i) and σ_(VOB).Examples of the function for calculating the correction coefficientinclude Expressions (3) and (4).correction coefficient α_(i)=β×σ_(VOB)/σ_(i)  (3)correction coefficient α_(i)=β×σ_(VOB)/√{square root over (σ_(i))}  (4)

Here, β is a constant that is determined arbitrarily. Also, in the casein which the value calculated using Expression (3) or (4) exceeds 1, thecorrection coefficient α_(i) is set to 1 since there is the risk ofover-correction if the correction coefficient exceeds 1. Note that theexpression for calculating the correction coefficient in the presentembodiment is merely an example, and the present invention is notlimited to this.

Next, in step S807 the correction value V_(i) for the i-th row isdetermined (i being a vertical coordinate, for example, i=0,1,2, . . .n). The correction value is calculated in accordance with Expression(5). Specifically, an average value is calculated by dividing theintegrated value calculated in step S803 by the number of data piecesused in the calculation of the integrated value, and then a blackreference level set in advance is subtracted from the average value. Theresult is then multiplied by the correction coefficient α_(i) determinedin step S806, thus obtaining the correction value for that row.correction value V _(i)=α_(i)×(S _(i)/number of data pieces−blackreference value)  (5)

Then, in step S808, correction is performed on an effective pixel unitin the i-th row in accordance with Expression (2), with use of thecorrection value V_(i).

The processing for the row ends when the effective pixel signalcorrection calculation has been performed through to the end of the row.Processing then returns to step S806, and this processing is repeatedthrough to the last row (step S809).

By performing the processing described above, a correction value thatreflects the variation in signals of the HOB pixels in the row isdetermined, thus enabling executing stripe noise correction withoutnewly increasing the amount of noise. Note that the above processing maybe performed on a PC (Personal Computer) instead of in a camera. Itshould also be noted that although examples are given in the embodimentsdescribed above in which the reference pixel areas have the samestructure as the aperture pixel area and are shielded, the presentinvention is not necessarily limited to this. For example, the referencepixels included in the reference pixel areas do not need to includephotodiodes. In such a case, the reference pixels do not need to beshielded.

As described above, according to the above embodiments, a determinationis made as to whether correction is to be performed, based on the valueof the standard deviation of pixel signals that is a reference, thusenabling suppressing the occurrence of new stripe noise in an image dueto over-correction. Also, horizontal stripe noise can be effectivelycorrected by changing the correction coefficient according to the valueof the standard deviation.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2009-114979, filed May 11, 2009, which is hereby incorporated byreference herein in its entirety.

The invention claimed is:
 1. An image capturing apparatus comprising: animage sensor having an effective pixel area composed of effective pixelsthat photoelectrically convert an object image, and a reference pixelarea composed of reference pixels that output pixel signals to be areference; a first calculation unit configured to calculate values of astatistical measure of the pixel signals output from the reference pixelarea; a second calculation unit configured to calculate integratedvalues with respect to each of rows of the pixel signals output from thereference pixel area; a third calculation unit configured to calculatecorrection values with respect to each of the rows by multiplyingcorrection coefficients to the integrated value for each of the rows,wherein the correction values are calculated based on the values of thestatistical measure; a correction unit configured to correct pixelsignals with respect to each of the rows output from the effective pixelarea with use of the correction values for each of the rows; and adetermination unit configured to determine whether correction is to beperformed by the correction unit, in accordance with the values of thestatistical measure.
 2. The image capturing apparatus according to claim1, wherein the reference pixel area is an optical black area composed ofshielded pixels that are shielded such that light does not enter.
 3. Theimage capturing apparatus according to claim 1, wherein the values of astatistical measure are standard deviations.
 4. A control method for animage capturing apparatus provided with an image sensor having aneffective pixel area composed of effective pixels that photoelectricallyconvert an object image, and a reference pixel area composed ofreference pixels that output pixel signals to be a reference, thecontrol method comprising the steps of: calculating values of astatistical measure of the pixel signals output from the reference pixelarea; calculating integrated values with respect to each of rows of thepixel signals output from the reference pixel area; calculatingcorrection values with respect to each of the rows by multiplyingcorrection coefficients to the integrated value for each of the rows,wherein the correction values are calculated based on the values of thestatistical measure; correcting the pixel signals with respect to eachof the rows output from the effective pixel area with use of thecorrection values for each of the rows; and determining whethercorrection is to be performed in the correction step, according to thevalues of the statistical measure.
 5. A control method according toclaim 4, wherein the values of a statistical measure are standarddeviations.